1. Field of the Invention
The present invention relates to a gas sampling device, particularly to one utilized in a Non-Dispersive Infrared (NDIR) gas concentration analysis instrument.
2. Related Prior Art
A "Non-Dispersive Infrared gas concentration analysis instrument" is operated in accordance with the principle that some specific gases have specific spectrums in the range of infrared frequency. For example, CO.sub.2 has a characteristic that a light at a wavelength of 4.3 micrometer will be absorbed thereby.
As shown in FIG. 1, a conventional gas sampling device comprises a columnar or square tube 51 which is used as a sampling means and is provided with many holes 52 on the wall thereof so as to permit gas to freely diffuse. The concentration of the gas existing, in the interior and exterior of the tube will reach a balanced condition via the holes due to the concentration difference and the principle of free diffusion. Moreover, a porous mesh 53 is provided on the outer surface of the tube to envelope the holes so as to prevent dust from entering the tube. A light source 54 and a light sensor 55 are also provided respectively on two sides of the tube for permitting, light emitted and detected at a specific range. In operation, the light to be emitted from the light source 54 goes through the interior of the tube and then is detected by the light sensor 55 which detects the intensity of the received infrared-light at a specific wavelength and outputs an electric signal representing the gas concentration.
However, since the diffusion speed of gas is proportional to the concentration difference, the increasing rate for the concentration of the gas in the tube will be faster at the beginning of sampling, and the diffusion rate will be slower as the concentration difference between the interior and the exterior of the tube reduces. Thus, it needs a longer time to reach a balance condition in a higher concentrated form.